Conventional piezoresistive pressure sensors are formed by a Wheatstone bridge that includes four piezoresistors. These four piezoresistors are placed near the edge of a deformable membrane, i.e., a diaphragm, where the stress change is high under external pressure. Of the four piezoresistors, two may be symmetrically positioned parallel to the adjacent diaphragm edge, and the other two may be symmetrically positioned normal, i.e., perpendicular, to the adjacent diaphragm edge. Under external pressure, the piezoresistors in parallel to the adjacent diaphragm edge are subjected to opposite stresses relative to the two piezoresistors that are normal to the adjacent diaphragm edge. This results in increased resistance for the piezoresistors that are normal and decreased resistance for the piezoresistors that are parallel. Accordingly, the output voltage of the Wheatstone bridge changes with external applied pressure.
In general, there are two types of pressure sensor designs based on Wheatstone bridges. One type of design places all four piezoresistors of the Wheatstone bridge proximate one edge of the diaphragm. The other type of design places one piezoresistor of the Wheatstone bridge on each of the four edges of the diaphragm. In either configuration, an electronic circuit detects the resistance changes of the piezoresistive bridge and outputs an electrical signal representative of the external applied pressure.
Increasing the pressure sensitivity of a pressure sensor may provide improved resolution and therefore yield improved device performance. Device sensitivity may be increased by increasing the size of a diaphragm. That is, a bigger diaphragm can provide higher deflection under a given applied external pressure and generate more change in stress at the piezoresistor locations. More change in stress at the piezoresistor locations produces a larger electrical output, therefore increased sensitivity. However, a larger diaphragm has the disadvantages of more fragile die/wafer, larger die size/higher cost, and degraded linearity performances.
Further, prior art piezoresistive pressure sensors can be very sensitive to process variations, especially the alignment of piezoresistor locations to the pressure cavity. That is, during fabrication, there can be a misalignment between the backside cavity of a pressure sensor and the piezoresistors formed in the diaphragm suspended across the cavity. Since the piezoresistors are placed symmetrically along the edge of the diaphragm, any cavity misalignment may translate to an asymmetry of the piezoresistors which can result in an output offset of the Wheatstone bridge.